1. Field of the Invention
The present invention relates to an electrophotographic plate and, more specifically, it relates to a double-layered electrophotographic plate with excellent durability.
2. Description of the Prior Art
Inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have been popularly used for the photosensitive layers of electrophotographic plates.
Studies on use of organic photoconductive materials typically represented by polyvinylcarbazole for the photosensitive layer have been advanced and several of them have been put to practical use. The organic photoconductive materials are advantageous over the inorganic photoconductive materials in that they are light in weight, can be made into films and fabricated into photosensitive bodies with ease. Further, since selenium and cadmium sulfide have to be recovered in view of their toxicity, increasing attention has been attracted more and more to the non-toxic organic photoconductive materials in recent years. The organic photoconductive materials, although having such advantages, have not hitherto been used so much as the electrophotographic plates, because they are inferior to the inorganic photoconductive materials in view of the sensitivity and the durability.
Recent endeavor has been devoted to the development of laminated type photosensitive bodies of a double layer structure consisting of a charge generating layer and a charge transporting layer, in which a function of generating charge carriers upon absorption of light and a function of transporting the charge carriers thus generated are sheared to each of the layers. As the result, high sensitivity organic electrophotographic plates have been put to practical use by combining respective organic compounds as a charge generator and as a charge transporting medium each having a high efficiency for the sheared function.
Usually, photoconductive organic dyes are employed as the charge generating layer and the polymers such as polyvinyl carbazole are used as the charge transporting layer for the double-layered electrophotographic plates. Recently, the charge transporting layers have often been prepared by dissolving electron-donating low molecular organic compounds, as the charge transporting medium, into insulating binder polymers. In this case, the performance such as the bondability with the lower layer, the surface hardness and the flexibility can be improved by the adequate selection for the binder polymers, whereby photosensitive bodies of excellent performance can be obtained.
Thus, the double-layered electrophotographic plates are advantageous in that the performance thereof can be improved by shearing various functions into each of the constituent layers but they still give rise to several problems.
In a double-layered electrophotographic plate, carriers generated upon absorption of light into charge generators in a charge generating layer are injected into and transported through a charge transporting layer. However, if traps are present due to impurities or the likes in the charge transporting layer, the carriers are caught in the traps to increase the residual potential and the carriers may sometimes be caught also at the interface between the charge generating layer and the charge transporting layer. Thus, the residual potential is gradually increased upon repeated use of the electrophotographic plate thereby tending to result in foggings in the photographic images. Such traps are provably formed, it is considered, due to the energy barrier at the interface between the charge generating layer and the charge transporting layer, the state of the interface, presence of the impurities in the constituent members such as the binder polymer and, further, due to the repeated exposure to electrical fields generated from corona discharge or to the light of imagewise exposure and cleaning lamps. In order to avoid such an increase in the residual potential, elimination of impurities in the material to be used or addition of various electron accepting compounds has been attempted. However, the former complicates the purification procedures and thus increases the production cost. While on the other hand, the latter often increases the dark decay, causes fluctuations in the surface potential upon repeated use, decreases the sensitivity and can not always provide a sufficient suppressing effect for the residual potential.